An Evolved UTRA or Long Term Evolution (LTE) system is currently being discussed by 3GPP, a standardization group for W-CDMA. LTE is a successor communication scheme to wideband code division multiple access (W-CDMA), high speed downlink packet access (HSDPA), and high speed uplink packet access (HSUPA). In the LTE system, orthogonal frequency division multiplexing (OFDM) is to be used for downlink and single-carrier frequency division multiple access (SC-FDMA) is to be used for uplink (see, for example, 3GPP TS36.211 (V8.0.0), September 2007).
In both downlink and uplink of the LTE system, one or more resource blocks are allocated to a user device (user equipment: UE) for communications. Resource blocks are shared by multiple user devices in the system. In LTE, a base station performs a process called scheduling to select user devices to which resource blocks are to be allocated in each subframe of 1 ms. A subframe may also be called a transmission time interval (TTI). One subframe includes multiple (e.g., two) slots. In downlink, the base station transmits a shared channel using one or more resource blocks to user devices selected in the scheduling. This downlink shared channel is called a physical downlink shared channel (PDSCH). In uplink, user devices selected in the scheduling transmit a shared channel using one or more resource blocks to the base station. This uplink shared channel is called a physical uplink shared channel (PUSCH).
In a communication system employing shared channels, it is necessary to signal (or report) allocation information of the shared channels to user devices for each subframe. A control channel used for this signaling in LTE is called a physical downlink control channel (PDCCH) or a downlink L1/L2 control channel. The physical downlink control channel (PDCCH) includes, for example, downlink scheduling information, acknowledgement information (ACK/NACK), an uplink scheduling grant, an overload indicator, and a transmission power control (TPC) command bit.
FIG. 1 shows an uplink frame structure and relationships between a subframe, slots, a PUSCH, and a PUCCH. An uplink L1/L2 control signal may be transmitted together with the physical uplink shared channel (PUSCH) using resources allocated to the PUSCH, or transmitted using resources dedicated to control signals. The uplink L1/L2 control signal to be transmitted together with the PUSCH includes scheduling information for the PUSCH. The uplink L1/L2 control signal to be transmitted using dedicated resources is called a physical uplink control channel (PUCCH). The physical uplink control channel includes, for example, a downlink channel quality indicator (CQI) and acknowledgement information (ACK/NACK) for the physical downlink shared channel. The CQI is used for scheduling and adaptive modulation and coding (AMC) of the physical downlink shard channel. The acknowledgement information includes either acknowledgement (ACK) indicating that the PDSCH has been properly received or negative acknowledgement (NACK) indicating that the PDSCH has not been properly received. A narrow frequency band is allocated to the PUCCH. However, since the PUCCH is transmitted according to a frequency hopping scheme in a subframe, the reception quality of the PUCCH is improved due to the frequency diversity gain.
If the proportion of the frequency band occupied by the PUCCH to the system frequency band is large, the amount of resources for transmitting the physical uplink shared channel (PUSCH) is reduced. Therefore, to improve the throughput, the proportion of the frequency band of the PUCCH to the system frequency band is preferably small. Meanwhile, users who are not transmitting the physical uplink shared channel (PUSCH) have to transmit CQIs and ACK/NACK via the PUCCH. To enable such users to smoothly report CQIs and ACK/NACK, it is preferable to increase the amount of radio resources for the PUCCH. Thus, it is necessary to enable a large number of users to efficiently use the PUCCH. However, methods for mapping CQIs and ACK/NACK and for multiplexing multiple users in the PUCCH have not been fully discussed yet.